Pulse generation in dual supply systems

ABSTRACT

Various apparatuses and methods are disclosed. The system describes a pulse generator comprising a first stage configured to be powered by a first voltage; and a second stage configured to be powered by a second voltage different from the first voltage, wherein the second stage is further configured to generate a pulse in response to an input to the first stage comprising a trigger and feedback from the second stage.

BACKGROUND

1. Field

The present disclosure relates generally to electronic circuits, andmore particularly, to pulse generation in dual supply systems.

2. Background

With the ever increasing demand for more processing capability in mobiledevices, low power consumption has become a key design requirement.Various techniques are currently employed to reduce power consumption insuch devices. One such technique involves reducing the operating voltageof certain circuits operating on a chip. As a result, different circuitson the chip operate at different voltages. Level shifters are used toconvert one voltage level to another voltage level. Level shifters allowa signal to pass from one voltage domain to another voltage domain.

A common circuit used today in dual voltage systems is a one-shot pulsegenerator. A pulse is generated in a first voltage domain by theone-shot and then level shifted to a second voltage domain. The pulse isgenerated by gating a trigger with a delayed version of the trigger. Thepulse-width is defined by the time between the trigger and the delayedtrigger. However, the delay circuit may not track well with process,voltage and temperature (PVT) variations. The pulse width can be verynarrow under extreme PVT conditions. This can cause a functional failureif the level-shifted pulse fails to switch from rail-to-rail. The onlyway to recover is by tuning the delay circuit. The can cost real estateand add timing complexities

SUMMARY

One aspect of a pulse generator includes a first stage configured to bepowered by a first voltage, and a second stage configured to be poweredby a second voltage different from the first voltage. The second stageis further configured to generate a pulse in response to an input to thefirst stage comprising a trigger and feedback from the second stage.

One aspect of a method includes generating a pulse from a pulsegenerator having a first stage powered by a first voltage and a secondstage powered by a second voltage different from the first voltage. Themethod includes generating a pulse in response to an input to the firststage comprising a trigger and feedback from the second stage.

Another aspect of a pulse generator includes pulse generating means forgenerating a pulse and pre-pulse generating means for generating apre-pulse from an input comprising a trigger and feedback from the pulsegenerating means. The pulse generating means is configured to generatethe pulse from the pre-pulse. The pre-pulse generating means isconfigured to be powered by a first voltage, and the pulse generatingmeans is configured to be powered by a second voltage different from thefirst voltage.

It is understood that other aspects of apparatuses and methods willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various aspects of apparatuses and methodsare shown and described by way of illustration. As will be realized,these aspects may be implemented in other and different forms and itsseveral details are capable of modification in various other respects.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of apparatuses and methods will now be presented in thedetailed description by way of example, and not by way of limitation,with reference to the accompanying drawings, wherein:

FIG. 1 is a functional block diagram illustrating an example of a pulsegenerator with feedback.

FIG. 2 is a schematic representation illustrating an example of a pulsegenerator with feedback.

FIG. 3 is a timing diagram illustrating an example of the operation of apulse generator with feedback.

FIG. 4 is a schematic representation illustrating another example of apulse generator with feedback.

DETAILED DESCRIPTION

Various aspects of the disclosure will be described more fullyhereinafter with reference to the accompanying drawings. This disclosuremay, however, be embodied in many different forms by those skilled inthe art and should not be construed as limited to any specific structureor function presented herein. Rather, these aspects are provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of the disclosure to those skilled in the art. Based on theteachings herein, one skilled in the art should appreciate that thescope of the disclosure is intended to cover any aspect of thisdisclosure, whether implemented independently of or combined with anyother aspect of the disclosure. For example, an apparatus may beimplemented or a method may be practiced using any number of the aspectsset forth herein. In addition, the scope of the disclosure is intendedto cover such an apparatus or method which is practiced using otherstructure and/or functionality in addition to or instead of otheraspects of this disclosure. It should be understood that any aspect ofthe disclosure disclosed herein may be embodied by one or more elementsof a claim.

Although particular aspects will be described herein, many variationsand permutations of these aspects fall within the scope of thedisclosure. Although some benefits and advantages of the preferredaspects are mentioned, the scope of the disclosure is not intended to belimited to particular benefits, uses, or objectives. Rather, aspects ofthe disclosure are intended to be broadly applicable to differentcircuits, technologies, systems, networks, and methods, some of whichare illustrated by way of example in the drawings and in the followingdescription. The detailed description and drawings are merelyillustrative of the disclosure rather than limiting, the scope of thedisclosure being defined by the appended claims and equivalents thereof.

The various circuits described throughout this disclosure may beimplemented in various forms of hardware. By way of example, any ofthese circuits, either alone or in combination, may be implemented as anintegrated circuit, part of an integrated circuit, discrete hardwarecomponents, or any other suitable implementation designed to perform thefunctions described herein. The integrated circuit may be an endproduct, such as a microprocessor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), programmable logic,memory, or any other suitable integrated circuit. Alternatively, theintegrated circuit may be integrated with other chips, discrete circuitelements, and/or other components as part of either an intermediateproduct, such as a motherboard, or an end product. The end product canbe any suitable product that includes integrated circuits, including byway of example, a cellular phone, a personal digital assistant (PDA), alaptop computer, a desktop computer (PC), a computer peripheral device,a multimedia device, a video device, an audio device, a globalpositioning system (GPS), a wireless sensor, or any other suitabledevice.

FIG. 1 is a functional block diagram illustrating an example of a pulsegenerator with feedback. The pulse generator 100 is shown with twostages. The first stage may be a latch 104 and the second stage may be alevel shifter 108. The latch 104 may be powered by a first voltagesource V_(DD1) and the level shifter 108 may be powered by a secondvoltage source V_(DD2). The first voltage source V_(DD1) may be greaterthan the second voltage source V_(DD2), or alternatively, second voltagesource V_(DD2) may be greater than the first voltage source V_(DD1). Inat least one embodiment, the pulse generator 100 may be configured tooperate with voltage scaling for dynamic power reduction, wherein thefirst and second voltage sources V_(DD1) and V_(DD2) may scaleindependently of each other according to the workload requirements ofthe system. By way of example, the first voltage source V_(DD1) mayscale below the second voltage source V_(DD2) if the system powered bythe first voltage source V_(DD1) has lower activity and needs to go intoa low power state, and vice versa.

The input to the latch 104 includes an external trigger 102 and feedback112 from the level shifter 108. In at least one embodiment, the externaltrigger 102 may be a clock or other periodic signal. As will bedescribed in greater detail later, the latch 104 generates a pre-pulse106 in the V_(DD1) domain in response to the input. The level shifter108 generates a pulse 110 in the V_(DD2) domain in response to thepre-pulse 106.

FIG. 2 is a schematic representation illustrating an example of a pulsegenerator with feedback. The operation of the pulse generator will bedescribed in connection with two logic states represented by two voltagebands: one near the supply voltage and one near the supply voltagereturn, typically ground. The term “high” may be used to reference theband near the supply voltage. By way of example, the term “high” used todescribe the operation of a circuit in the V_(DD1) domain means that thevoltage is in a band near the supply voltage V_(DD1). The same appliesfor the V_(DD2) domain. The term “low” may be used to reference the bandnear the supply voltage return or ground.

Returning to FIG. 2, the pulse generator 100 is shown with a latch 104and a level shifter 108. As described earlier, the latch 104 provides ameans for generating a pre-pulse 106 from an input comprising anexternal trigger 102 and feedback 112 from the level shifter 108, andthe level shifter 108 provides a means for generating a pulse 110 fromthe pre-pulse 106.

In at least one embodiment, the latch 104 is configured as an SR latchwith a gated output. In this embodiment, the SR latch 202 is constructedfrom a pair of cross-coupled NOR gates 204, 206. Specifically, theoutput of the NOR gate 204 is coupled to a first input of the NOR gate206 and the output of the NOR gate 206 is coupled to a first input ofthe NOR gate 204. An external trigger is coupled to a second input ofthe NOR gate 206 and feedback from the level shifter 108 is coupled to asecond input of the NOR gate 204. The output from the NOR gate 204 isthe Q output of the SR latch 202 and the output from the NOR gate 206 isthe complimentary Q* output of the SR latch 202. In operation, theexternal trigger 102 is used to set the SR latch 202 (i.e., force the Qoutput high) and the feedback 112 from the level shifter 108 is used toreset the SR latch 202 (i.e., force the Q output low). In at least oneembodiment, the sizing of the NOR gate 204 may be skewed for strong NMOSand weak PMOS transistors so that the NOR gate 204 acts as another levelshifter between the feedback 112 in the V_(DD2) domain and the Q outputof the NOR gate 204 in the V_(DD1) domain.

With both the external trigger 102 and the feedback 112 from the levelshifter 108 low, the cross-coupling between the two NOR gates 204, 206maintains the state of the SR latch 202. The SR latch 202 is set bydriving the external trigger 102 high with the feedback 112 from thelevel shifter 108 low. The SR latch 202 remains set when the externaltrigger returns to low. Similarly, the SR latch 202 is reset by drivingthe feedback 112 from the level shifter 108 high with the externaltrigger 102 low. The SR latch 202 remains reset when the feedback 112from the level shifter 108 returns to low.

In the embodiment shown, the complimentary output Q* from the SR latch202 is gated with external trigger 102. Specifically, the Q* output fromthe SR latch 202 is coupled to a first input of a NOR gate 208 and theexternal trigger 102 is coupled to a second input of the NOR gate 208.The output of the NOR gate 208 is the pre-pulse 106A that is provided tothe level shifter 108. The pre-pulse 106 is also provided to an inverter210. The inverter 210 provides an inverted pre-pulse 106B to the levelshifter 108. As will be explained in greater detail later, the levelshifter 108 generates a pulse 110 in a different voltage domain from thepre-pulse 106A and the inverted pre-pulse 106B.

In operation, the rising edge of the external trigger 102 is used to setthe SR latch 202. The Q* output from the SR latch 202 is forced low whenthe SR latch 202 is set. With the Q* output from the SR latch 202 low,the NOR gate 208 is enabled. Specifically, the NOR gate 208 acts as aninverter for the external trigger 102 when the Q* output from the SRlatch 202 is low, thereby setting the pre-pulse 106 high with thefalling edge of the external trigger 102. The level shifter 108generates a pulse 110 in the V_(DD2) domain from the pre-pulse 106A (andthe inverted pre-pulse 106B) generated in the V_(DD1) domain. Thetransition of the leading edge of the pulse 110 forces the feedback 112provided to the SR latch 104 high. The feedback resets the SR latch 202and forces the Q* output high. The Q* output, in turn, disables the NORgate 208, which forces the pre-pulse 106 low regardless of the state ofthe external trigger 102.

The level shifter 108 may take on various forms depending upon theparticular application and design requirements. In at least oneembodiment, the level shifter 108 may be implemented as a CMOS levelshifter. The CMOS level shifter includes a pair of NMOS transistors 212,214 with their sources coupled to ground, a pair of PMOS transistors220, 222 with their sources coupled to V_(DD2), and a pair ofcross-coupled PMOS transistors 216, 218. The PMOS transistor 216 has asource coupled to the drain of the PMOS transistor 220 and a draincoupled at a node N1 to the drain of the NMOS transistor 212. The pulse110 generated by the level shifter 108 is output from the node N1 andprovides a voltage swing between V_(DD2) and ground. The PMOS transistor218 has a source coupled to the drain of the PMOS transistor 222 and adrain coupled at a node N2 to the drain of the NMOS transistor 214. Thegate of the PMOS transistor 216 is coupled to the node N2 and the gateof the PMOS transistor 218 is coupled to the node N1.

The level shifter is operated in the V_(DD2) domain and does not haveaccess to the V_(DD1) domain other than the pre-pulse be converted. Thepre-pulse 106A is coupled to the NMOS transistor 212 and the invertedpre-pulse 106B is coupled to the NMOS transistor 214.

When the pulse generator 100 is inactive, the pre-pulse 106A is low andthe inverted pre-pulse 106B is high. In this state, the pre-pulse 106Aturns on the PMOS transistor 216 and turns off the NMOS transistor 212.The inverted pre-pulse 106B turns off the PMOS transistor 222 and turnson the NMOS transistor 214. The node N2 is pulled down to ground throughthe NMOS transistor 214, which turns on the PMOS transistor 216. Thepulse 110 output from the level shifter 108 at node N1 is pulled up toV_(DD2) through the PMOS transistors 216, 220, which turns off the PMOStransistor 218. The pulse 110 is also provided to an inverter 224 in thelevel shifter 108. The inverter 224 is used to provide the invertedpulse as feedback 112 to the latch 104.

As discussed earlier in connection with the latch 104, the pre-pulse106A is forced high with the falling edge of the external trigger 102.With the pre-pulse 106A high, the PMOS transistor 220 is turned off andthe NMOS transistor 212 is turned on. The inverted pre-pulse 106B turnson the PMOS transistor 222 and turns off the NMOS transistor 214. Thepulse 110 output from the level shifter 108 at the node N1 is pulleddown to ground through the NMOS transistor 212, which turns on the PMOStransistor 218. The node N2 is then pulled up to V_(DD2), through thePMOS transistors 218, 222, which turns off the PMOS transistor 216. Theinverted pulse, which is high, is provided by the inverter 224 to thelatch 104 as feedback 112 to reset the SR latch 202 and force thepre-pulse 106A low. This ensures that the pre-pulse 106 remains highuntil after the pulse 110 generated by the level shifter 108 has fullytransitioned from V_(DD2) to ground.

Once the pre-pulse 106A is forced low by the feedback 112, the levelshifter is forced back into its inactive state with the PMOS transistor220 turned on and the NMOS transistor 212 turned off by the pre-pulse106A, and the PMOS transistor 222 turned off and the NMOS transistor 214turned on by the inverted pre-pulse 106B. The node N2 is pulled down toground through the NMOS transistor 214, which turns on the PMOStransistor 216. The pulse 110 output from the level shifter 108 at nodeN1 is pulled back up to V_(DD2) through the PMOS transistors 216, 220,which turns off the PMOS transistor 218. The inverted pulse is forcedlow and provided to the latch as feedback 112.

The inverter 224 provides both a means for inverting the pulse 110 toprovide feedback to the latch 104 and a means for delaying the feedback.Additional delay elements (not shown) may be added in the feedback pathto increase the delay, and thereby increase the width of the pulse 110.The delay elements may be a series of inverters or other devices.

FIG. 3 is a timing diagram illustrating an example of the operation of apulse generator. The pre-pulse 106A is generated by the latch 104 inresponse to the falling edge of the external trigger 102. The pre-pulse106A is output from the latch 104 to the level shifter 108 to generatethe pulse 110. In this example, the pulse 110 is triggered by the risingedge of the pre-pulse 106A. The inverted pulse is provided to the latchas feedback 112. Specifically, once the pulse 110 fully transitions fromV_(DD2) to ground, the feedback signal 112 is forced high. As describedearlier, the feedback to the latch 104 may be delayed by a series ofdelay elements in the feedback path to increase the width of the pulse110. The feedback is used to reset the SR latch by forcing the Q outputlow, which in turn forces the complementary Q* output from the SR latchhigh. The rising edge of the Q* output forces the pre-pulse 106A back toa low state, which in turn forces the pulse 110 back to a high state.The feedback signal 112 is forced low with the rising edge of the pulse110. The SR latch is then set with the next rising edge of externaltrigger 102, which forces the Q output high and the Q* output low.

The SR latch may be used when the pulse is passed from a high voltagedomain to a low voltage domain. However, when the pulse is being passedfrom a low voltage domain to a high voltage domain, the SR latch may beomitted. FIG. 4 is a schematic representation illustrating an example apulse generator having this embodiment. The pulse generator 100 is shownwith two stages. The first stage may be a NOR gate 208 and inverter 210and the second stage may be a level shifter 108. The level shifter 108may be the same as described earlier and is reproduced here in FIG. 4.Alternatively, the level shifter 108 may take on another form. In anyevent, the pulse 110 is inverted by the inverter 224 and provideddirectly to the first input of the NOR gate 208 as feedback 112. Theexternal trigger 102 is provided to the second input of the NOR gate208. The output from the NOR gate 208 is coupled to the level shifter108 as the pre-pulse 106A. The output is also provided to the inverter210 which is used to provide an inverted pre-pulse 106B to the levelshifter 108.

When the pulse generator 100 is inactive, the pulse 110 output from thelevel shifter 108 is high and the feedback 112 provided to the NOR gate208 is low. The low feedback signal 112 enables the NOR gate 208 to actas an inverter for the external trigger 102. When the external trigger102 transitions from a high state to a low state, the pre-pulse 106Aoutput from the NOR gate 208 is forced high and the inverted pre-pulse106B is forced low. As explained in greater detail earlier, this forcesthe pulse 110 at the output of the level shifter to transition fromV_(DD2) to ground. Once the pulse 110 transitions, the feedback providedto the NOR gate 208 is forced high. The high feedback signal disablesthe NOR gate 208 forcing the pre-pulse 106 output low regardless of thestate of the external trigger 102. When the pre-pulse 106 transitionslow, the pulse 110 output from the level shifter 108 is forced back intothe high state.

The various aspects of this disclosure are provided to enable one ofordinary skill in the art to practice the present invention. Variousmodifications to exemplary embodiments presented throughout thisdisclosure will be readily apparent to those skilled in the art, and theconcepts disclosed herein may be extended to other magnetic storagedevices. Thus, the claims are not intended to be limited to the variousaspects of this disclosure, but are to be accorded the full scopeconsistent with the language of the claims. All structural andfunctional equivalents to the various components of the exemplaryembodiments described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe claims. Moreover, nothing disclosed herein is intended to bededicated to the public regardless of whether such disclosure isexplicitly recited in the claims. No claim element is to be construedunder the provisions of 35 U.S.C. §112(f), unless the element isexpressly recited using the phrase “means for” or, in the case of amethod claim, the element is recited using the phrase “step for.”

What is claimed is:
 1. A pulse generator, comprising: a first stageconfigured to be powered by a first voltage; and a second stageconfigured to be powered by a second voltage different from the firstvoltage, wherein the second stage is further configured to generate apulse in response to an input to the first stage comprising a triggerand feedback from the second stage, wherein the pulse transitions from afirst state to a second state in response to an edge of the trigger andthen transitions from the second state to the first state in response tothe same edge of the trigger before a subsequent edge of the triggeroccurs.
 2. The pulse generator of claim 1 wherein the second stagecomprises an inverter configured to invert the pulse generated by thesecond stage, and wherein the feedback from the second stage to theinput of the first stage comprises the inverted pulse.
 3. The pulsegenerator of claim 1 wherein the second stage comprises a delay elementconfigured to delay the feedback from the second stage to the firststage, wherein the width of the pulse generated by the second stage is afunction of the delay.
 4. The pulse generator of claim 1 wherein thefirst stage is further configured to generate a pre-pulse output fromthe trigger and the feedback, and wherein the second stage comprises alevel shifter configured to level shift the pre-pulse to generate thepulse.
 5. The pulse generator of claim 4 wherein the pre-pulse generatedby the first stage comprises a leading edge responsive to the triggerand a trailing edge responsive to the feedback from the second stage. 6.The pulse generator of claim 4 wherein the first stage comprises a NORgate configured to gate the trigger and the feedback to generate thepre-pulse.
 7. The pulse generator of claim 1, wherein the pulse has amagnitude substantially equal to the second voltage, and wherein thefirst stage is configured to generate a pre-pulse as an input to thesecond stage, and the pre-pulse has a magnitude substantially equal tothe first voltage.
 8. The pulse generator of claim 1, wherein the firststage comprises a latch configured to be set by the trigger and reset bythe feedback.
 9. A method of generating a pulse from a pulse generatorcomprising: a first stage configured to be powered by a first voltageand a second stage configured to be powered by a second voltagedifferent from the first voltage, the method comprising: generating apulse in response to an input to the first stage comprising a triggerand feedback from the second stage, wherein the pulse transitions from afirst state to a second state in response to an edge of the trigger andthen transitions from the second state to the first state in response tothe same edge of the trigger before a subsequent edge of the triggeroccurs.
 10. The method of claim 9 wherein the generating of the pulsecomprises inverting the pulse generated by the second stage, wherein thefeedback from the second stage to the input of the first stage comprisesthe inverted pulse.
 11. The method of claim 9 wherein the generating ofthe pulse comprises delaying the feedback from the second stage to thefirst stage, wherein the width of the pulse generated by the secondstage is a function of the delay.
 12. The method of claim 9 wherein thegenerating of the pulse comprises generating a pre-pulse from thetrigger and the feedback, and level shifting the pre-pulse to generatethe pulse.
 13. The method of claim 12 wherein the pre-pulse generated bythe first stage comprises a leading edge responsive to the trigger and atrailing edge responsive to the feedback from the second circuit. 14.The method of claim 12 wherein the generating of the pulse comprisesgating the trigger and the feedback with a NOR gate to generate thepre-pulse.
 15. A pulse generator, comprising: pulse generating means forgenerating a pulse; pre-pulse generating means for generating apre-pulse from an input comprising a trigger and feedback from the pulsegenerating means; wherein the pulse generating means is configured togenerate the pulse from the pre-pulse, and wherein the pulse transitionsfrom a first state to a second state in response to an edge of thetrigger and then transitions from the second state to the first state inresponse to the same edge of the trigger before a subsequent edge of thetrigger occurs; and wherein the pre-pulse generating means is configuredto be powered by a first voltage, and the pulse generating means isconfigured to be powered by a second voltage different from the firstvoltage.
 16. The pulse generator of claim 15 wherein the pulsegenerating means comprises means for inverting the pulse, and whereinthe feedback from the pulse generating means to the input of thepre-pulse generating means comprises the inverted pulse.
 17. The pulsegenerator of claim 16 wherein the pulse generating means comprises meansfor delaying the feedback from the pulse generating means to thepre-pulse generating means, wherein the width of the pulse generated bythe pulse generating means is a function of the delay.
 18. The pulsegenerator of claim 15 wherein the pulse generating means comprises alevel shifter configured to level shift the pre-pulse to generate thepulse.
 19. The pulse generator of claim 15 wherein the pre-pulsegenerated by the pre-pulse generating means comprises a leading edgeresponsive to the trigger and a trailing edge responsive to the feedbackfrom the pulse generating means.
 20. The pulse generator of claim 15wherein the pre-pulse generating means comprises a NOR gate configuredto gate the trigger and the feedback to generate the pre -pulse.
 21. Apulse generator, comprising: a first stage configured to be powered by afirst voltage; and a second stage configured to be powered by a secondvoltage different from the first voltage, wherein the second stage isfurther configured to generate a pulse in response to an input to thefirst stage comprising a trigger and feedback from the second stage,wherein the pulse transitions from a first state to a second state andtransitions from the second state to the first state in response to anedge of the trigger, wherein the first stage is further configured togenerate a pre-pulse output from the trigger and the feedback, whereinthe second stage comprises a level shifter configured to level shift thepre-pulse to generate the pulse, and wherein the first stage comprisesan SR latch configured to be set by the trigger and reset by thefeedback, and a NOR gate configured to gate an output from the SR latchand the trigger to generate the pre-pulse.
 22. A method of generating apulse from a pulse generator comprising: a first stage configured to bepowered by a first voltage and a second stage configured to be poweredby a second voltage different from the first voltage, the methodcomprising: generating a pulse in response to an input to the firststage comprising a trigger and feedback from the second stage, whereinthe pulse transitions from a first state to a second state andtransitions from the second state to the first state in response to anedge of the trigger, wherein the generating of the pulse comprisesgenerating a pre-pulse from the trigger and the feedback, and levelshifting the pre-pulse to generate the pulse, and wherein the generatingof the pre-pulse comprises setting an SR latch by the trigger, resettingthe SR latch by the feedback, and gating an output from the SR latch andthe trigger with a NOR gate to generate the pre-pulse.
 23. A pulsegenerator, comprising: pulse generating means for generating a pulse;pre-pulse generating means for generating a pre-pulse from an inputcomprising a trigger and feedback from the pulse generating means;wherein the pulse generating means is configured to generate the pulsefrom the pre-pulse, and wherein the pulse transitions from a first stateto a second state and transitions from the second state to the firststate in response to an edge of the trigger; and wherein the pre-pulsegenerating means is configured to be powered by a first voltage, and thepulse generating means is configured to be powered by a second voltagedifferent from the first voltage, and wherein the pre-pulse generatingmeans comprises an SR latch configured to be set by the trigger andreset by the feedback, and a NOR gate configured to gate an output fromthe SR latch and the trigger to generate the pre-pulse.